(1) Field of the Invention
The present invention relates to boiling water reactors (BWR), particularly, to preferable fuel assemblies and reactor cores for labor-saving fuel shuffling operation by increasing size of the fuel assembly and reducing number of the fuel assemblies with ensuring thermal margin and reactor shut down margin.
(2) Description of the Prior Art
A fuel assembly for BWR is, in general, composed of a bundle of fuel rods forming a square lattice, each of the fuel rods is manufactured by inserting a plurality of fuel pellets containing fissile material into a cladding tube and sealing, and a channel box having a hollow square cross section, an outer side of which is about 14 cm, which covers the above fuel bundle. A reactor core is formed in a cylindrical shape by further bundling of the above fuel assemblies. As for fuel, enriched uranium or/and plutonium-enriched uranium is used in a chemical form of oxide.
As reactivity of reactor core decreases with burning of fuel, the fuel is loaded into the reactor core more than the critical amount at beginning of the reactor operation cycle so that the reactor maintains criticality. Excess reactivity yielded by loading of the excess fuel is controlled by adjusting neutron absorption in the reactor core with mixing burnable poison such as gadolinia etc. into the fuel, and inserting control rods having cruciform cross section, which comprise boron carbide or hafnium, among a plurality of adjacent fuel assemblies.
In order to allow inserting the cruciform control rod into the reactor core, water gap regions being filled with non-boiling water, of which gap size is almost twice of a blade thickness of the control rod, are provided around the channel box of the fuel assemblies. Moreover, water rods filled with non-boiling water are provided at center of the fuel assembly in view of neutron flux flattening. Atomic numbers ratio of hydrogen to uranium in the reactor core average (optionally it is called H/U ratio hereinafter) which depends on a size of the above non-boiling water region and the amount of fissile material is adjusted in a range of 4-5 in order to make necessary enrichment of the fuel lowest mainly in view of uranium resource saving.
On the other hand, the excess reactivity increases at shut down of the reactor because of increase in water by phase change of steam to water. Accordingly, it becomes important to ensure reactor shut down margin. Regarding to methods for increasing reactor shut down margin of the fuel assembly and the reactor core, the following two methods are well known as prior art;
(1) JP-A-63-231293 (1988)
In accordance with this prior art, neutron average energy in the reactor core is reduced, a difference in neutron moderating effect between upper portion and lower portion of the reactor core is reduced, and consequently, the reactor shut down margin is increased, by making a ratio of transverse cross section area of the water gap region which is a saturated water region outside the channel box to transverse cross section area of pellets in all fuel rods in the channel box at least one.
(2) JP-A-2-12088 (1990)
In accordance with this prior art, an excess reactivity of the reactor core is reduced and, consequently, the reactor shut down margin is increased, by composing the fuel assembly so as to have a non-boiling water region of which area is at least 9.1% of the transverse cross section area of the channel box.
Hitherto, increase of output power has been achieved in general by increasing in the number of fuel assemblies. However, the increase in the number of fuel assemblies in the reactor core causes increase in the numbers of fuel assemblies to be shuffled and to be transferred in periodical inspection of the reactor core, and consequently, necessary period and man-hour for fuel exchange operations increase and an utilization factor for the plant can be lowered. Therefore, there is a problem that a scale merit which is expected by the increase of the output power is not necessarily obtained. Accordingly, in view of labor saving for fuel exchange operation, it is effective to increase size of a fuel assembly for reducing total number of fuel assemblies in the reactor core.
On the other hand, the increase in size of a fuel assembly causes increment of local power peaking factor in a diametral direction because of increase in heterogeneity of the reactor core. Further, the number of the fuel assemblies in the reactor core decreases by increasing size of the fuel assembly under a condition that the reactor core has a constant size. Accordingly, the number of control rods being inserted among the fuel assemblies also decreases. It means relative decrease in total length of the control rod blade, reducing in control rods worth, and decrease in the reactor shut down margin. Therefore, it is necessary to have means for preventing above described problems when increasing size of the fuel assembly.
When the above described prior art are applied for increasing size of the fuel assembly, the following defects exist;
In accordance with the prior art, JP-A-63-231293 (1988), the reactor shut down margin can be increased by reducing the neutron moderating effect, but thermal margin is decreased by increase in the ratio of the transverse cross section area of the water gap region to the transverse cross section area of the total fuel pellets.
In accordance with the prior art, JP-A-2-12088 (1990), the transverse cross section of the non-boiling water region is defined by taking the internal transverse cross section of the channel box as a base. Therefore, there are some cases in which effective increment of the reactor shut down margin can not be achieved depending on a loading condition of the fuel rods in the fuel assembly. As for the thermal margin, the situation is the same. Further, the above defined value for the transverse cross section of the non-boiling water region varies depending on the kind of the fuel material such as uranium-plutonium mixed oxides, or enriched uranium.